Method for chlorine plasma modification of silver electrodes

ABSTRACT

A method for the modification of a surface of a silver electrode wherein the surface is treated with a chlorine plasma is described. Control of the power, flux density and timing provide the manner to modify the silver surface, and so implant the chlorine atoms and ions into the silver. The present invention provides a method to produce thin-film silver electrodes with a very controlled surface. Such electrodes can provide quantitative quality of measurement. The present invention extends to a method for the modification of a surface of any metal wherein the surface is treated with a plasma.

[0001] This invention relates to the modification of metal electrodesand in particular to the chlorine plasma modification of silverelectrodes.

[0002] Silver electrodes coated with silver chloride layers (Ag/AgClelectrodes) have for many years formed the basis for medical electrodesdue to the excellent charge transfer characteristics and nonpolarisability of the silver chloride material. The main applications ofAg/AgCl electrodes concern amperometric (current sensitive) andpotentiometric (voltage sensitive) sensors. Indeed, Ag/AgCl layers haveformed the basis for many electrocardigram (ECG) electrodes and havebeen incorporated into both sensing devices and reference electrodes.

[0003] One common way of producing Ag/AgCl electrodes is theelectrochemical technique whereby chloride ions are deposited on thesilver surface reacting therewith to form a silver chloride layer(AgCl).Sintered Ag/AgCl electrodes have been used in the past as well asserigraphic inks. The latter is known as “thick film” technology andcombines a mixture of silver and silver chloride particles trapped in apolymer. A common alternative to these methods is chemical chloriding,which is a technique applied in both thin and thick film technology.

[0004] The thicker the film of silver chloride (AgCl) formed on thesilver electrode the less sensitive the electrode becomes. As such, thinfilm technology is becoming an essential key of sensing and bio-sensing,due to the concept of miniaturisation. Increasingly microscopic scaledevices allow for more accurate readings. Therefore, Ag/AgCl electrodeshave been designed and adapted to thin-film technology in order to beused as micro-sensors.

[0005] However, the present thin film methods of producing Ag/AgClelectrodes in general have been found not to be satisfactory. Thetechnique of chemical chloriding thin films is a delicate task which isdifficult to control and often results in the formation of a silverchloride film which is too thick and does not have uniform propertiesthroughout. The thicker the silver chloride layer the higher theelectrical impedance of the electrode which reduces the sensitivity.Also, chemical chloriding simply involves the surface absorption ofchlorine on the silver electrode and as such does not always provide anAgCl film with a strong adhesion to the surface of the silver electrode.

[0006] Depositing chloride ions on the silver electrode usingelectroplating is also a difficult process to control and the resultingAg/Cl layer is often be too thick and lacking in uniformity. It has alsobeen shown that this technique does not provide good adhesion of theAgCl layer to the silver electrode and also induces surface cracking,both of which reduce the effectiveness of the electrode.

[0007] The present invention is directed to overcoming these problemsand provides a method which can more closely control the formation andthickness of the silver chloride layer. The method also provides asilver chloride layer which is more uniform and stable in structure,thereby providing a more sensitive, stable and accurate reading.

[0008] According to a first aspect of the present invention there isprovided a method for the modification of a surface of a silverelectrode wherein the surface is treated with a chlorine plasma.

[0009] Control of the power, flux density and timing provide the mannerto modify the silver surface, and so implant the chlorine atoms and ionsinto the silver. Different control parameters provide different silversurfaces, e.g. thicker or thinner, as desired or necessary.

[0010] Preferably, the silver electrode is in the form of a silverthin-film. More preferably, the silver electrode is fabricated by theevaporation of silver onto a glass plate to form a silver thin-film. Theglass plate is preferably cleaned and coated with a thin metallicsub-layer prior to the addition of silver. Preferably, the sub-layer isa chromium or nichrome sub-layer and is coated on the glass plate usinga sputtering system.

[0011] Typically, the sputtering system can be a Nordiko NM200/RFG1250sputtering system using an inert plasma and a chromium target.Preferably, the inert plasma is argon plasma.

[0012] The modification of the surface of the silver electrode can takeplace in a plasma chamber. A surface of the silver electrode isbombarded with the chlorine plasma. Preferably, the modification of thesilver electrode takes place in a reaction ion etching (RIE)radio-frequency (RF) plasma chamber. Preferably, the chamber is pumpeddown prior to the modification of the silver electrode.

[0013] According to another aspect of the present invention there isprovided a method for the modification of a surface of a metal whereinthe surface is treated with a plasma.

[0014] Preferably, the plasma is in the form of a stream and is formedfrom a reactive gas. More preferably, the reactive gas is a halogen suchas chlorine gas.

[0015] The modification of the metal surface can take place in a plasmachamber. A surface of the metal is bombarded with the plasma stream.Preferably, the modification of the metal takes place in a reaction ionetching (RIE) radio-frequency (RF) plasma chamber. Preferably, thechamber is pumped down prior to the modification of the metal.

[0016] The method is suitable, for example, for the treatment of metalelectrodes and in particular for the treatment of silver electrodes inthe form of a thin-film, possibly fabricated by the evaporation of themetal onto a glass plate.

[0017] The invention extends to any metal item, e.g. an electrode,having a surface modified as herein described.

[0018] The invention will be more clearly understood by way ofdescription thereof with reference to the accompanying drawings inwhich:

[0019]FIG. 1(a) is a graph of the impedance spectrum of silver thin-filmbefore treatment according to the present invention;

[0020]FIG. 1(b) is a graph of the impedence spectra of silver thin-filmafter electrochloriding in a 0.5M KCl solution; FIG. 1(c) is a graph ofthe impedence spectra of silver thin-film after treatment according tothe present invention;

[0021]FIG. 2(a) is a SEM view of the surface of silver thin-film afterelectrochloriding (x 2.5k);

[0022]FIG. 2(b) is a SEM view of the surface of silver thin-film aftertreatment according to the present invention (x 15k);

[0023]FIG. 3 is a graph of EDX measurements realised on silver thin-filmshowing spectra of Ag (white) and AgCl (red);

[0024]FIG. 4 is a graph of XPS spectra of silver thin-film modifiedaccording to the present invention;

[0025]FIG. 5 are graphs showing Ag 3d5/2 and Ag 3d5/2 binding energyshifts; and

[0026]FIG. 6 are graphs showing Cl 2p3/2 and Cl 2p3/2 binding energyshifts.

[0027] To aid in the understanding and advantages of the presentinvention, experimental testing was undertaken on a silver electrodemodified according to the present invention (2) and compared toidentical testing on a non-coated silver electrode, and on a silverelectrode with chloride ions deposited thereon using the technique ofelectroplating (1).

[0028] For the purposes of this experiment, all the silver electrodesused were fabricated by evaporating silver in a form 99.99% pure(forexample, 99.99% purity, Goodfellow, Cambridge U.K. type silver)to form asilver thin-film on top of a BDH plain microscope glass slide.Evaporation was carried out using an electron-beam chamber (LeyboldL560E⁻ Beam), at an evaporation rate of 100 Å/s⁹, under high vacuum(2.10⁻⁵ mbar). Prior to the evaporation stage, the glass sides werecleaned and then introduced into a Nordiko NM200/RFG1250 sputteringsystem, using an argon plasma and a chromium target. A sub-layer ofchromium was deposited on the glass slide and this improved the adhesionof the silver thin-film to the glass slide when deposited thereon. Thesilver electrodes thus formed (electrode samples) were then furthertreated as follows:

[0029] (1) Electrochemical Chloriding—Electroplating

[0030] A portion of the electrode samples were encapsulated in anelectrochemical cell, allowing an active electrode surface of 0.96 cm².The electrodes were dipped into a 0.5M potassium chloride (KCl)solution. An anodic current of 0.98 mAmps was applied for several timedurations (5s, 7s, 10s, 15s, 20s and 25s). The chloride ions in the KC1solution migrated to the positive silver electrode and were depositedthereon. The chloride ions reacted with the silver and formed a layer ofsilver chloride (AgCl). The electrochemical chloriding was performedusing AutoLab™ instrumentation, for example a General PurposeElectrochemical System (GPES) and Frequency Response Analyser (FRA).

[0031] (2) Plasma Modification

[0032] Another portion of the electrode samples were modified accordingto the present invention with a chlorine plasma stream. The plasmastream used was a pure chlorine plasma stream, in a Reaction Ion Etching(RIE) radio-frequency (RF) plasma chamber. Typical operating parametersfor the plasma chamber are given below in Table 1. TABLE 1 1 to 100mTorr: 5 to 500 Watts (per cm²): 1 to 50 sccms (standard cm³) ofchlorine: 1 ms to 5 seconds plasma time

[0033] Operating Parameters for preparing, for example, a low impedanceelectrode for a suitable system, are:

[0034] 10-20 mTorr

[0035] ˜170 Watts 5-10 sccms

[0036] 1 second plasma time

[0037] Experimental Testing

[0038] The above treated electrode samples (electrochemical electrodeand, plasma electrode) and the standard non-treated electrodes were thensubjected to the following tests:

[0039] Electrochemical tests;

[0040] X-Ray Photoelectron Spectroscopy

[0041] (XPS) investigations;

[0042] Energy Dispersive X-Ray Spectroscopy (EDX) analysis; and

[0043] Scanning Electron Microscopy (SEM) analysis.

[0044] All the electrochemical tests and measurements were carried outwith an AutoLab apparatus (GPES and FRA). A 5 mV AC voltage was appliedusing a range of frequency from 10,000 Hz to 0.1 Hz for all these testsand measurements were carried out in a phosphate buffer solution (PBS),to which 0.9% NaCl was added.

[0045] XPS investigations were carried out on a KRATOS XSAM 800apparatus. The X-ray source was run with MgKα X-rays at 240W (13.8 kV,18 mA).

[0046] SEM pictures were taken from a SEM/EDX system, an Hitachi S3200Nmicroscope interfaced with an Oxford Instrument Link ISIS EDXspectrometer.

[0047] As a standard, the non-treated electrode samples were firstcharacterised by electrochemistry and SEM. The silver thin-films (Ag TF)deposited by evaporation technique have a very smooth surface and reactpoorly with the electrolyte (large impedance), as shown in FIG. 1(a).

[0048] After electrochemical chloriding of the electrode samples, SEManalysis shows large granules at the surface of the silver layer, givinga very rough surface topography. Islands of silver chloride have grown,generating a non-uniform surface as shown in FIG. 2(a) at which theelectrolyte can react easily. Once the Ag/AgCl reduction/oxidationcouple is formed, a nearly non-polarisable electrode is obtained. Thepresence of AgCl layer which acts as a bridge between the electrode andthe electrolyte and the roughening of the electrode surface are directlylinked to the overall electrode impedance. As shown in FIG 1(b) areduced impedence of a factor of 100 is realised for the sampleelectrodes treated in this manner (electrochemical electrode). Theimpedence of the electrochemical electrode was tested at intervals of 12and 15 seconds and show that the impedence of this electrode increaseswith time which is an indication that the AgCl layer is not stable.

[0049] For the electrode samples treated according to the presentinvention (plasma electrodes) with a chlorine plasma stream, a similarreduction in impedence is also observed. This treatment apparentlycreated a singular physical reaction between the silver layer and thechlorine atoms, molecules and ions present within the plasma. Theimpedance is also lower than that of the sample electrodes by a factorof 100 as shown in FIG. 1(c), and this can be achieved only if a“bridge” behaving compound, able of exchanging charges with theelectrolyte, is created.

[0050] SEM analysis of the plasma electrode show pictures of a“cauliflower” like structure similar to that of the electrochemicalelectrode. However, in the case of a plasma electrode the layer formedthereon is thinner, more uniform and more controllable. The layerso-formed includes what appears to be a combination of an AgCl film, achloride form of chlorine (Cl) and individual chlorine atoms which areembedded in the surface of the silver.

[0051] The similarity in behaviour and structure obtained after the twotreatments (1) and (2)above possibly suggest that a similar AgCl filmwas created in each case. However, EDX measurements of the plasmaelectrode, shown in FIG. 3, show the presence of remaining chlorineatoms in the surface of the silver electrode, which supports thefeasibility of creating an AgCl film using this treatment.

[0052] XPS measurements were carried out on several sample electrodestreated with plasma streams other than chlorine plasma in order todetermine the chemical nature and physical structure of the AgCl thinfilm formed from the second treatment process. The plasmas used werefrom similar reactive gases such as fluorine (Fl) as AgCl can beapproximated to behave as any other form of silver halide; theirelectronic structure being very similar. The XPS results show energypeaks related to the Ag 3d and the Cl 2p electronic orbits as shown inFIG. 4.

[0053] The shifts in energy are related to changes within theseelectronic orbits. First of all, as shown by EDX measurements chlorinepeaks on spectra show that the surface modification compound remains atthe surface of the electrode sample after plasma treatment. Then, bothchlorine and silver peaks are shifted towards lower energies. FIGS. 5and 6 compare the value of these peaks to pure silver and chlorinetheoretical peaks, as referred in literature [ref. Web+XPS Handbook].Silver halides (AgI and AgF) Ag 3d_(5/2) peaks show a ±0.27eV shiftcompared to pure silver. The silver peak of the plasma treated sampleelectrode is shifted by about ±0.4 for the same Ag 3d_(5/2) peak. Theenergy shift spread is similar to the other silver halides tested. AgClcan be expected to behave in a similar manner to other forms of silverhalide as the electronic structure of each are very similar.Nevertheless, the exact values can not be deducted using the energy peakvalues of the other silver halides, but does provide a close enough ideaof the expected energy shift range. Furthermore, quantification of thedifferent elements existing at the surface of the silver thin-film,showed that silver and chlorine are the main species (respectively 75%to 80% and 12% to 17%), and only very few contaminants are present (i.e.carbon and oxygen). Thus, only few oxide forms of silver must becreated, leaving the shift in the peaks under the influence of thechlorine ions.

[0054] Chlorine was also detected in a chloride form, as the Cl 2p_(3/2)peaks are within the same region of binding energy as the alkalichloride, as shown in FIG. 5, with an energy shift of about −0.25eV.Even if this chloride shift is compared to a non-metal chloride form,one can still assume that the structure of AgCl and NaCl are closeenough to give a very similar electronic distribution of the molecularorbits (M.O.), and hence an analogous shift value.

[0055] Discussion

[0056] These results support the idea that the chlorine ions within theplasma, react chemically with the silver surface and create a halidesilver/alkali chloride structure type.

[0057] The texture and macro-appearance of the plasma electrodes are notsimilar to the electrochemical electrodes and this is clearly shown bycomparison of FIG. 2(a) and FIG. 2(b). The plasma electrodes gave a mattyellowish colour, as described for abnormally electrochloridedelectrodes in Crenner et al. When increasing the chloriding time for agiven input power and chlorine flux, the colour of the sample electrodesturn from a white/yellowish shade to a brown/plum tint. It has beenreported (Yalcinkaya and Power (1997), Janz and Ives (1968)and Beck andRice (1984)) that the latter tint is characteristic of a furtherreduction of AgCl to free interstitial Ag, and that “the whiteallotropic form of AgCl does not correspond to the standard state forcrystalline silver chloride”. Actually, Neuman Michael (1995) 2^(nd)edition reported that “pure silver chloride is amber coloured andbecomes dark grey because of fine particles of silver”. These roughoptical observations are consistent with the fact that none or poorcrystalline AgCl would be expected to grow at the surface of the thinfilm, producing a matt yellowish colour. Further exposure to thechlorine plasma, which is highly reactive, could be susceptible toinduce silver sputtering or etching (as it can be observed on the SEMpictures). Therefore, it is possible that an increasing number of silverparticles could be trapped within the created silver chloride, leadingto a darker colour specific to the interstitial silver.

[0058] The present invention provides a method for providing silvermodification by chlorine, to produce thin-film silver electrodes with avery controlled surface. Such electrodes can provide quantitativequality of measurement.

1. A method for the modification of a surface of a silver electrodewherein the surface is treated with a chlorine plasma.
 2. A method asclaimed in claim 1 wherein the silver electrode is in the form of asilver thin-film.
 3. A method as claimed in claim 2 wherein theelectrode is fabricated by an evaporation of silver onto a glass plate.4. A method as claimed in claim 3 wherein the glass plate is coated witha thin metallic sub-layer prior to the addition of silver.
 5. A methodas claimed in claim 4 wherein the sub-layer is a chromium or nichrome.6. A method as claimed in claim 5 wherein the sub-layer is coated on theglass plate using a sputtering system.
 7. A method for the modificationof a surface of a metal wherein the surface is treated with a plasma. 8.A method as claimed in claim 7 wherein the plasma is formed from areactive gas.
 9. A method as claimed in any one of the preceding claimswherein the method is carried out in a plasma chamber.
 10. A method asclaimed in claim 9 wherein the plasma chamber is a reaction ion etchingradio-frequency plasma chamber.
 11. A method as claimed in claim 9 orclaim 10 wherein the plasma chamber is pumped down prior to modificationof the silver electrode or metal surface.
 12. A method as claimed inclaim 11 wherein the operating pressure in the plasma chamber is withinthe range 1 to 100 mTorr.
 13. A method as claimed in any one of thepreceding claims wherein the operating power is in the range of 5 to 500Watts (per cm²).
 14. A method as claimed in any one of the precedingClaims wherein the operating flux density method is within the range 1to 50 sccms of gas.
 15. A method as claimed in claim 14 wherein the gasis chlorine.
 16. A method as claimed in any one of the preceding Claimswherein the operating time is within the range 1 ms to 5 seconds plasmatime.
 17. A silver electrode whenever obtainable by a method as claimedin any one of claims 1 to
 16. 18. A modified metal surface wheneverobtainable by a method as claimed in any one of claims 7 to
 16. 19. Asilver electrode having a silver chloride surface layer provided byplasma modification.
 20. A metal surface having a plasma modifiedsurface.